1. Field of the Invention
The present invention relates to the field of radio-frequency (RF) signal distribution, and in particular to an apparatus and method for distributing RF signals through low bandwidth infrastructures for re-transmission by antennas, wherein additional signals for controlling the antennas are transmitted over same low bandwidth infrastructures.
2. Description of Prior Art
The need for efficient and low-cost systems for distributing radio frequency (RF) signals has continued to increase as the demand for wireless communications has rapidly grown. Distribution of RF signals is particularly difficult and expensive in areas with many natural and man-made obstacles which scatter or absorb RF radiation. For example, the problem of RF distribution is especially acute inside and around building structures.
Current in-building distribution systems consist of two major parts. The first is a set of antennas and associated accessories required for re-transmitting the RF signals inside a building. The second is a cabling system, e.g., an optical fiber network, used for interconnecting the in-building antennas with a main antenna. The latter is usually installed on top of the building or at some location where the external RF signals can be easily received. In cases where the RF signals are supplied via a high transmission bandwidth cable the main antenna can be replaced by a direct interface with the RF network, e.g., in the basement.
The cost of installing and maintaining such in-building distribution networks is very high. One of the major cost factors is the cabling network. Not only must cable for carrying RF signals be installed but another set of cables and wiring is needed to incorporate any monitoring and diagnostic features into the system, e.g., the ability to control the gain of the re-transmitting antennas or check if the antennas are operational from a centralized building location. In addition, running new cables between various rooms, floors, or wings of a building is usually time-consuming and disruptive. For these reasons, desirable solutions to in-building RF distribution systems should incur minimal installation cost, require no special tooling (as necessitated, e.g., in fiber optic networks), and produce no undue disturbance in the building during installation and operation. It would also be advantageous for such distribution networks, including monitoring and diagnostic subsystems, to be consistent or implementable with common in-building cable infrastructure.
The most effective manner of satisfying these criteria would be to use an existing or standard in-building cable infrastructure. To find in a typical building a pre-exiting cable which can transmit RF signals bi-directionally, let alone accomplish this and simultaneously be used to monitor antenna functionality, may seem especially surprising to most. The only conceivable candidates for pre-existing media for consideration would be standard in-building cabling such as unshielded or shield twisted pair (UTP and STP) used for local area networks (LAN), telephone cables, multi-mode optical fiber links, and power lines.
Unfortunately, several obstacles must be overcome to make use of standard in-building cabling. The major problem is related to the frequency bands used for transmitting RF information signals. Cellular communications presently utilize a carrier frequency around 1 GHz. For, example, the AMPS protocol uses the bandwidth from 824-894 MHz and GSM is transmitted between 890-960 MHz. Recent legislation has allowed PCS services to move to even higher frequencies (e.g., 1,850-1,990 MHz). In comparison, the standard in-building unshielded or shielded twisted pair cables are limited to much lower transmission bandwidths.
The most common standard cable category 5 (10 base T) UTP cable, for example, has signal loss and cross talk properties that limit the bandwidth to approximately 0-100 MHz for distances &lt;100 m. Although these parameters suffice for LAN applications, they are clearly inadequate for the delivery of cellular and PCS signals to and from remote antenna sites. This is especially unfortunate in that 10 base T cable contains four twisted pair cables each of which, as suggested by the parent application cited above, could otherwise be used to transmit signals for radio communication or for antenna monitoring and control.
Prior art solutions, therefore, employ wide bandwidth media such as coaxial cables and optical fibers. These media have to be installed separately, and require specially trained personnel, as discussed above.
Thus, the challenge is to transmit high frequency RF signals over the standard low bandwidth infrastructures, especially 10 base T cable. The common method of accomplishing this goal is to initially down-convert the band of the RF signal to an intermediate frequency (IF) which is within the bandwidth of the cable. Then, the IF signal is fed through the standard low bandwidth cable found in the building. At the remote antenna site the IF signal is up-converted to recover the original RF signal and the recovered RF signal is re-transmitted by the remote antenna. This solution is illustrated in FIG. 1 and will be discussed in the detailed description.
A major problem encountered in implementing this solution involves the stability of local oscillators. These provide the reference signals required by the mixers to down-convert and up-convert the signals. To ensure proper operation the local oscillators must generate a stable tone at the selected high RF frequency (e.g., 800 MHz). It is critical that the frequency of the two oscillators be matched to within at least the channel spacing of the RF signals. In fact, it is desirable that the oscillators be "locked" to each other to preserve the frequency of the RF signal band. This issue becomes even more crucial at higher frequencies, e.g., the PCS bandwidth centered around 2 GHz where the relative width of the communication channels is small in comparison to the carrier frequency.
The two solutions to this problem are to either use very stable oscillators (e.g., &lt;1 part per million stability), which are prohibitively expensive, or to distribute the oscillator tone from a central location. The second option is not viable either, since the media under consideration does not have the bandwidth required for the implementation of such a system.
The existing solutions to distributing a stable oscillator tone are limited. In U.S. Pat. No. 5,046,135 Hatcher shows how to eliminate frequency instabilities in a receiver frequency converter due to inherent local oscillator instability by adding a marker signal at the down-conversion stage. The marker signal is distorted in the same manner as the IF signal and a second stage down-converter computes this distortion by comparison with the marker signal before undertaking any further down-conversion.
This solution is complicated, since it breaks down the conversion process into two steps and requires the addition of a marker tone in addition to the oscillator frequencies and the signal. Moreover, it cannot be employed in conjunction with the low bandwidth media found in buildings. Indeed, the main purpose of the invention is to gradually and reliably down-convert very high-frequency signal received, e.g., from satellites in orbit.
U.S. Pat. No. 4,959,862 issued to Davidov et al. addresses a novel scheme for the delivery of FM modulated subcarriers over a fiber-optic link for cable television transmission (CATV). Conventional CATV systems use vestigal sideband amplitude modulation (VSB-AM) for transmission of analog video channels to home users. In comparison, frequency division multiplexed frequency modulated (FDM-FM) signals can provide a higher signal to noise ratio and a longer transmission distance. Davidov et al. describe a method for the conversion of VSB-AM channels to FDM-FM channels before transmission over the fiber-optic link. After transmission, the FM signals are re-converted back to AM signals before transmission to the home. A 4 MHz "global reference" is distributed along with the FM signals to AM signals.
Although Davidov et al. address the idea of a global signal which can be used for reference ("locking") of conversion stages, this idea is inapplicable to the problem at hand. First, the reference signal is high frequency and is distributed to the remote antenna sites for the purpose of FM to AM signal conversion. It is not a signal which is compatible with a system based on a limited and low bandwidth medium for transmitting RF signals. In fact, Davidov et al. emphasize the fact that this system uses a fiber-optic medium which is broadband. Moreover, in Davidov's system architecture it is not necessary to use the global reference, rather it is provided for convenience. The only advantage Davidov et al. derive from using a centralized oscillator is the reduction of oscillator phase noise.
In U.S. Pat. No. 5,109,532 Petrovic et al. discuss the transmitter and receiver of a radio communication link. This link requires up- and down-conversion of the signals to be transmitted to and from the radio band of interest. The frequency and phase of the oscillators used for up- and down-conversion are a large cost and performance consideration.
The problem is solved by adding a radio frequency pilot tone to the up-converted signals before transmission. At the receiver, a local oscillator is used to down-convert both the RF signal and the pilot tone. Any phase or frequency deviations of the local oscillator affect the RF signal and the pilot tone equally. Therefore, both signals can be used to cancel the phase and frequency variations, resulting in a clean recovered signal. This cancellation method solves the problem of local oscillator stability at the receiver.
Although the disclosure is intended to solve a similar problem as the present invention, namely the stability of a remote oscillator, the method by which the problem is solved is quite different. Furthermore, the method does not describe, nor is it obvious, how one would implement this technique over a low-bandwidth medium, since the pilot tone is at a RF frequency.
In addition to devising a system for proper "locking" of oscillators to be able to transmit RF signals through low bandwidth infrastructure there are additional unsolved problems. In a typical RF distribution system multiple remote antennas re-transmit the up-converted RF signal. To ensure complete coverage the coverage areas of the individual antennas overlap. Thus, a user will frequently receive signals from multiple antennas simultaneously. When the individual oscillators used for the up-conversion at those antennas are not exactly frequency matched the user will hear a baseband tone or beat at the difference between the frequencies of the two local oscillators.
Thus, efficient and reliable distribution of RF signals over low bandwidth infrastructures remains an unsolved problem.